USRP Hardware Driver and USRP Manual: Python API

| USRP Hardware Driver and USRP Manual Version: 4.9.0.makepkg-0-g9ec1f582 UHD and USRP Manual |

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Python API

UHD supports a Python API, in case the C++ or C APIs are not the right solution for your application.

# Installing the Python API

In order to install the Python API when building UHD from source, make sure you have the CMake variable ENABLE_PYTHON_API set to ON (e.g., by running cmake -DENABLE_PYTHON_API=ON). UHD requires Python header files in order to compile the Python API. On most Linux systems, there are packages called "python3-dev" or "python3-devel" that provide that functionality. On Windows, these headers always get installed when using the binary installers provided on https://www.python.org/downloads/windows/.

If CMake can't find the Python headers or library, specify the PYTHON_INCLUDE_DIR and/or PYTHON_LIBRARY CMake variables manually.

Note that since UHD version 4.0.0.0, Python 2 is no longer supported.

## Installing on Windows

Static linking is unsupported on Windows. Otherwise, compiling the Python API on Windows is no different from other operating systems. Only the build output differs. On Windows the Python API is built using poetry tooling as a Python binary distribution package (wheel). It is located in the dist directory of the build output. The preferred way to install the Python Package is using poetry install command, assuming <python> as the Python executable in your specific Python Environment where poetry is available.

    cd <build directory>\python
    <python> -m poetry install

Alternatively, you can install the Python package using the pip install command. For example:

    <python> -m pip install <build directory>\python\dist\uhd-4.x.x-cp3xx-cp3xx-win_amd64.whl

For installation of the Python API package from the Python Package Index (PyPI) please refer to section Python Packages under page Binary Installation.

Python 3.8 Note: If you receive an error similar to this when running import uhd in Python 3.8 and above on Windows:

    ImportError: DLL load failed while importing libpyuhd: The specified module could not be found.

this indicates a problem finding one or more of the DLLs that the UHD Python module depends on to load correctly.

Python 3.8 includes a change to the paths Windows searches when attempting to find a module's dependent DLLs. The UHD python package attempts using os.add_dll_directory to add the directory for uhd.dll to the DLL search path. First, it will try based on the UHD_PKG_PATH environment variable that will be created by the NSIS Windows installer, and if that fails, it will try to use a variable configured by CMake based on the CMAKE_INSTALL_PREFIX variable. If both fail, you will need to use os.add_dll_directory to add the path of uhd.dll to the DLL search path. Please also consider the troubleshooting notes in section Python Packages.

If UHD was built with USB support, you will need to add the path of libusb-1.0.dll to the DLL search path before importing the UHD package. You can do this by copying the libusb-1.0.dll to the same directory as the uhd.dll. Alternatively, you can add the DLL to the search path by using os.add_dll_directory in your python script.

## Advanced Usage Notes

UHD uses the PyBind11 library to generate its Python bindings. UHD ships its own copy of PyBind11, in order to facilitate the access to that library, as it is not packaged for many operating systems, but also to lock down its version. For the purpose of experimentation, it is, however possible to replace the version of PyBind11 shipped with UHD by overriding the PYBIND11_INCLUDE_DIR CMake variable.

# Using the Python API

The Python API mirrors the C++ API, so the C++ reference manual can be used to understand the behaviour of the Python API as well.

Names in the Python API have been modified to follow a PEP8-compatible naming convention, for example, uhd::usrp::multi_usrp in C++ corresponds to uhd.usrp.MultiUSRP in Python (this makes UHD/Python code implicitly compatible with most linters, but it also has the side-effect of hiding symbols that get imported from the C++ domain). The following two snippets are equivalent. First the C++ version:

#include <uhd/usrp/multi_usrp.hpp>
// ...
auto usrp = uhd::usrp::multi_usrp::make("type=b200");
usrp->set_rx_freq(100e6);
uhd::usrp::multi_usrp::make
static sptr make(const device_addr_t &dev_addr)
multi_usrp.hpp

Now the Python version:

import uhd
# ...
usrp = uhd.usrp.MultiUSRP("type=b200")
usrp.set_rx_freq(100e6)

Not all API calls from the C++ API are also supported in the Python API, and the Python API has some additional functions that are not available in C++, but for the most part, the uhd::usrp::multi_usrp API is identical.

# One-off transmit/receive applications

A common type of Python-based SDR applications are those which produce or consume a limited number of samples. For example, an application could receive a second's worth of samples, then do offline processing, print the result, and exit. For this case, convenience API calls were added to the Python API. The following snippet is an example of how to store 1 second of samples acquired at 1 Msps:

import uhd
def recv_to_file():
"""RX samples and write to file"""
usrp = uhd.usrp.MultiUSRP("type=b200")
num_samps = 1e6
if not isinstance(args.channels, list):
args.channels = [args.channels]
samps = usrp.recv_num_samps(
1e6, # Number of samples
2.4e9, # Frequency in Hz
1e6, # Sampling rate
[0], # Receive on channel 0
80, # 80 dB of RX gain
)
samps.tofile('samples.dat')

This kind of API is particularly useful in combination with Jupyter Notebooks or similar interactive environments.

# Thread Safety and the Python Global Interpreter Lock

From the Python wiki page on the GIL:

> In CPython, the global interpreter lock, or GIL, is a mutex that protects access to Python objects, preventing multiple threads from executing Python bytecodes at once.

During some performance-critical function calls, the UHD Python API releases the GIL, during which Python objects have their contents modified. The functions calls which do so are uhd::rx_streamer::recv, uhd::tx_streamer::send, uhd::tx_streamer::recv_async_msg and uhd::find. To be clear, the streamer functions listed here violate the expected contract set out by the GIL by accessing Python objects (from C++) without holding the GIL. This is necessary to achieve rates similar to what the C++ API can provide.

To this end, users must ensure that the Python objects accessed by the listed functions are handled with care. In simple, single threaded applications, this won't require any extra work. However, in more complicated and/or multi- threaded applications, steps must be taken to avoid thread-unsafe behavior. For example, if an application needs to call recv() in one thread, and access the sample buffer from another thread, a synchronization method (ie. a mutex) must be used to safeguard access to that buffer.